Astronomers are hunting for “wandering” black holes in dwarf galaxies, a pursuit that could unlock clues about the rapid growth of supermassive black holes in the early universe. These smaller galaxies offer a unique laboratory for studying black hole formation and evolution, potentially revealing a “fossil record” of the seeds from which these cosmic giants grew.
The challenge stems from a discrepancy in current cosmological models. The James Webb Space Telescope (JWST) has discovered supermassive black holes existing remarkably early in the universe’s history – less than one billion years old. Traditional theories suggest that the processes of galactic mergers and accretion needed to grow these behemoths should take over a billion years to reach such sizes. This has led scientists to explore the possibility of “black hole seeds” that provided a head start to the growth process.
These seeds are categorized as either “heavy” or “light,” and have been difficult to identify in the early universe. However, models predict that signatures of these seeds should be observable in dwarf galaxies in the local universe, galaxies with stellar masses billions of times that of our sun. Dwarf galaxies are particularly valuable for this research because their relatively quiet merger histories, compared to larger galaxies, provide a clearer record of their evolution.
The research, utilizing NASA’s Hubble Space Telescope and Chandra X-ray Observatory, focuses on identifying Active Galactic Nuclei (AGNs) – regions where a black hole is actively accreting matter – that are not located at the center of their host galaxy. This “off-nuclear” activity is a key indicator of a wandering black hole.
“Compared to more massive galaxies, dwarf galaxies can have lower central stellar densities and more irregularly shaped potential wells,” explained Megan R. Sturm of Montana State University to Space.com. “if a black hole forms in the outer reaches of its host galaxy, it is unlikely to ever spiral down into the center. Some researchers have even predicted that approximately half of all black holes in dwarf galaxies are wandering.”
Detecting these wandering black holes is complicated by the need to distinguish their signals from other sources of radiation, such as star formation regions and supernova explosions. Astronomers address this by observing potential candidates across multiple wavelengths of light.
The team investigated 12 dwarf galaxies previously identified as hosting AGNs via radio wave detection. Eight of these AGNs appeared offset from the galactic center, suggesting they could be harboring wandering black holes. However, further investigation revealed a nuance: one of these sources, designated ID 64, was actually a much more distant AGN that simply appeared to align with the dwarf galaxy from our perspective.
“Generally, supermassive black holes reside in the nucleus of massive galaxies,” Sturm said. “However, eight of the dwarf galaxies in our sample displayed compact radio emission originating from outside of the optical nucleus of the galaxy, offset by around one to two kiloparsecs [approximately 3,262 light-years] and in some cases outside of the host galaxy entirely. These are potentially ‘wandering’ black hole candidates.”
The remaining seven candidates require further scrutiny to determine if they truly host wandering black holes or if the radio signals originate from more distant sources. The team believes the JWST, with its enhanced capabilities, could be instrumental in resolving this ambiguity.
“Identifying the origin of the off-nuclear radio sources for the remaining seven wandering black hole candidates may be possible with the exquisite capabilities of the JWST,” Sturm explained. “With higher resolution, JWST could potentially observe the source of the compact radio emission, whether it be the core of a disrupted dwarf galaxy/star cluster within the host galaxy or a background, high-redshift galaxy.”
The team’s findings, published in the Astrophysical Journal, highlight the potential of dwarf galaxies as crucial sites for understanding the origins of supermassive black holes and the processes that shaped the early universe. The search for these wandering black holes continues, promising to refine our understanding of these enigmatic objects and their role in cosmic evolution.
